While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology.
One technology that has attracted a good deal of attention recently utilizes as the deep UV light source a high-intensity KrF excimer laser, especially an ArF excimer laser featuring a shorter wavelength. There is a desire to have a microfabrication technique of finer definition by combining exposure light of shorter wavelength with a resist material having a higher resolution.
In this regard, the recently developed, acid-catalyzed, chemical amplification type resist materials are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance. The chemical amplification type resist materials include positive working materials that leave the unexposed areas with the exposed areas removed and negative working materials that leave the exposed areas with the unexposed areas removed.
In chemical amplification type, positive working, resist compositions to be developed with alkaline developers, an alkali-soluble phenol or a resin and/or compound in which carboxylic acid is partially or entirely protected with acid-labile protective groups (acid labile groups) is catalytically decomposed by an acid which is generated upon exposure, to thereby generate the phenol or carboxylic acid in the exposed area which is removed by an alkaline developer. Also, in similar negative working resist compositions, an alkali-soluble phenol or a resin and/or compound having carboxylic acid and a compound (crosslinking agent) capable of bonding or crosslinking the resin or compound under the action of an acid are crosslinked with an acid which is generated upon exposure whereby the exposed area is converted to be insoluble in an alkaline developer and the unexposed area is removed by the alkaline developer.
On use of the chemical amplification type, positive working, resist compositions, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (to be referred to as photoacid generator) in a solvent, applying the resist solution onto a substrate by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask of a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed area of the resist film, obtaining a positive pattern profile. The substrate is then etched by any desired technique. Finally the remaining resist film is removed by dissolution in a remover solution or ashing, leaving the substrate having the desired pattern profile.
The chemical amplification type, positive working, resist compositions adapted for KrF excimer lasers generally use a phenolic resin, for example, polyhydroxystyrene in which some or all of the hydrogen atoms of phenolic hydroxyl groups are protected with acid labile protective groups. Iodonium salts, sulfonium salts, and bissulfonyldiazomethane compounds are typically used as the photoacid generator. If necessary, there are added additives, for example, a dissolution inhibiting or promoting compound in the form of a carboxylic acid and/or phenol derivative having a molecular weight of up to 3,000 in which some or all of the hydrogen atoms of carboxylic acid and/or phenolic hydroxyl groups are protected with acid labile groups, a carboxylic acid compound for improving dissolution characteristics, a basic compound for improving contrast, and a surfactant for improving coating characteristics.
Bissulfonyldiazomethanes as shown below are advantageously used as the photoacid generator in chemical amplification type resist compositions, especially chemical amplification type, positive working, resist compositions adapted for KrF excimer lasers because they provide a high sensitivity and resolution and eliminate poor compatibility with resins and poor solubility in resist solvents as found with the sulfonium and iodonium salt photoacid generators.

Although these photoacid generators are highly lipophilic and highly soluble in resist solvents, they have poor affinity to or solubility in developers so that upon development and/or resist removal, the photoacid generators can be left on the substrate as insoluble matter (consisting of the photoacid generator or a mixture thereof with the resin).
For example, upon development, the resist material which has poor affinity to or solubility in a developer deposits on developed spaces in the exposed area or on lines in the unexposed area as foreign matter.
JP-A 3-103854 discloses bis(4-methoxyphenylsulfonyl)diazomethane as a photoacid generator having a methoxy group introduced therein. As long as we confirmed, the methoxy group is not fully effective. The photoacid generator is often left on the substrate as insoluble matter (consisting of the photoacid generator or a mixture thereof with the resin) upon development and/or resist film removal.
If unsubstituted bis(phenylsulfonyl)diazomethane or bis(cyclohexylsulfonyl)diazomethane having alkyl groups instead of aryl groups is used in a resist material as the photoacid generator for reducing lipophilic property, resolution is deteriorated. If it is added in large amounts, the problem of insoluble matter upon development and/or resist film removal remains unsolved.
Aside from the countermeasure for foreign matter, JP-A 10-90884 discloses to introduce such an acid labile group as t-butoxycarbonyloxy, ethoxyethyl or tetrahydropyranyl into disulfonediazomethane for the purpose of improving the contrast of positive resist material. We empirically found that these compounds are unstable and ineffective for eliminating the foreign matter upon development and resist film removal.
Searching for a countermeasure to the foreign matter problem, we already synthesized sulfonyldiazomethanes having an acyl group (e.g., acetyl) or methanesulfonyl group introduced therein and found that they were useful as the photoacid generator in chemical amplification type resist composition. Since these arylsulfonyldiazomethanes having an acyl group or methanesulfonyl group introduced therein lack stability under basic conditions during their synthesis, the yield of diazo formation is sometimes low. See JP-A 2001-055373 and JP-A 2001-106669.
It is known from JP-A 8-123032 to use two or more photoacid generators in a resist material. JP-A 11-72921 discloses the use of a radiation-sensitive acid generator comprising in admixture a compound which generates a sulfonic acid having at least three fluorine atoms upon exposure to radiation and a compound which generates a fluorine atom-free sulfonic acid upon exposure to radiation, thereby improving resolution without inviting nano-edge roughness and film surface roughening. JP-A 11-38604 describes that a resist composition comprising an asymmetric bissulfonyldiazomethane such as a bissulfonyldiazomethane having alkylsulfonyl and arylsulfonyl groups or a bissulfonyldiazomethane having arylsulfonyl and alkoxy-substituted arylsulfonyl groups and a polyhydroxystyrene derivative having acid labile groups as the polymer has a resolution at least comparable to prior art compositions, a sufficient sensitivity and significantly improved heat resistance. However, we empirically found that these resist compositions are unsatisfactory in resolution and in the effect of eliminating the foreign matter on the pattern upon development. From the synthetic and industrial standpoints, it is difficult to obtain bilaterally asymmetric bissulfonyldiazomethanes.
Aside from the above-discussed problem of insoluble matter upon development and/or removal, there is also a problem that the pattern profile often changes when the period from exposure to post-exposure baking (PEB) is prolonged, which is known as post-exposure delay (PED). Such changes frequently reveal as a slimming of the line width of unexposed areas in the case of chemical amplification type positive resist compositions using acetal and analogous acid labile groups, and as a thickening of the line width of unexposed areas in the case of chemical amplification type positive resist compositions using tert-butoxycarbonyl (t-BOC) and analogous acid labile groups. Since the period from exposure to PEB is often prolonged for the operational reason, there is a desire to have a stable resist composition which is free from such changes, that is, has PED stability.
In some resist processes, baking is performed at far higher temperatures (e.g., 130° C.) than the customary baking temperature of 120° C. or below as disclosed in JP-A 6-266112. In this case, the bissulfonyldiazomethanes shown above by structural formulae can be thermally decomposed to generate acids due to their low heat resistance so that acidolysis takes place everywhere regardless of whether the areas are exposed or unexposed, failing in pattern formation.
The solubility of photosensitive agents or photoacid generators was the problem from the age when quinonediazide photosensitive agents were used in non-chemical amplification type resist materials. Specific considerations include the solubility of photoacid generators in resist solvents, the compatibility of photoacid generators with resins, the solubility (or affinity) of photo-decomposed products after exposure and PEB and non-decomposed compound (photoacid generator) in a developer, and the solubility of the photoacid generator and photo-decomposed products thereof in a remover solvent upon resist removal or peeling. If these factors are poor, there can occur problems including precipitation of the photoacid generator during storage, difficulty of filtration, uneven coating, striation, abnormal resist sensitivity, and foreign matter, left-over and staining on the pattern and in spaces after development.
The photoacid generator in resist material is required to meet a fully high solubility in (or compatibility with) a resist solvent and a resin, good storage stability, non-toxicity, effective coating, a well-defined pattern profile, PED stability, and no foreign matter left during pattern formation after development and upon resist removal. The conventional photoacid generators, especially diazodisulfone photoacid generators do not meet all of these requirements.
As the pattern of integrated circuits becomes finer in these days, a higher resolution is, of course, required, and the problem of foreign matter after development and resist removal becomes more serious.